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Abstract

We demonstrate a tunable and omnidirectional microlaser in the form of a microdroplet of a dye-doped, cholesteric liquid crystal in a carrier fluid. The cholesteric forms a Bragg-onion optical microcavity and the omnidirectional 3D lasing is due to the stimulated emission of light from the dye molecules in the liquid crystal. The lasing wavelength depends solely on the natural helical period of the cholesteric and can be tuned by varying the temperature. Millions of microlasers can be formed simply by mixing a liquid crystal, a laser dye and a carrier fluid, thus providing microlasers for soft-matter photonic devices.

Figures (5)

Fig. 1 The schematic view of the arrangement of CLC molecules in a cholesteric micro-droplet with parallel anchoring of the LC molecules at the surface. The helical structure of the liquid crystal originates from the center of the droplet and gives rise to concentric shells of constant refractive index. This dielectric structure is optically equivalent to the well-known Bragg-onion optical microcavity.

Fig. 2 (a) A typical cholesteric droplet with a pitch p = 2.2 μm in glycerol. The light and dark concentric shells are due to the spatial variation of the refractive index of the cholesteric liquid crystal in the radial direction. (b) Close up of the center of the cholesteric droplet, when viewing in the direction parallel to the disclination line. (c) Cholesteric droplet with PBG in the visible range of light, viewed under crossed polarizers and white-light illumination. (d–f) (
Media 1) Omnidirectional (3D) lasing in a cholesteric droplet illuminated by laser pulses (λ = 532 nm) and a weak white background illumination. (d) Below the lasing threshold (1.6 mJ/cm2) the droplet is fluorescing uniformly. (e) Just at the threshold for lasing (1.9 mJ/cm2), a bright spot of radiating monochromatic light can be observed in the center of the droplet. (f) Lasing becomes very intense at a high pump power (12 mJ/cm2).

Fig. 3 Lasing characteristics of a single droplet of dye-doped CLC. (a) The spectra of light emitted from the center of the CLC microdroplet at different energies of the pumping pulse. (b) The radiated laser-light intensity as a function of the input-pulse energy density. The threshold for lasing is clearly seen at ∼1.8 mJ/cm2. (c) Magnified lasing spectrum showing a laser linewidth of ∼0.10 nm. (d) The threshold for lasing as a function of the diameter of the CLC microdroplet. All the spectra were measured using an imaging spectrometer with a 0.05 nm resolution (Andor, Shamrock SR-500i) and cooled EM-CCD camera (Andor, Newton DU970N).

Fig. 4 Lasing spectrum of a single CLC droplet compared to the reflection spectrum of a 30 μm planar cell filled with the same CLC mixture. The reflection spectrum was measured for light propagating along the helix of the CLC.

Fig. 5 (a) Lasing intensity from a single 50 μm CLC droplet as a function of the angle of rotation of the photodetector around the axis of the cylindrical tube, containing the micro-droplets. (b) Lasing spectra as a function of temperature. At higher temperatures the laser line is shifted outside the optimum wavelength region of the dye used, so the laser emission ceases.